The alignment calibration of deployable components on a spacecraft, such as reflectors, antennas and other payloads, is generally carried out during an in-orbit test (IOT) in order to meet on-station precision pointing requirements. Deployable reflectors, such as shown in FIG. 1, can have large in-orbit alignment errors due to the lack of deployment repeatability. A typical deployment error, for example, can be up to an unacceptable 0.4°.
Currently, many spacecraft operate with a beacon sensor (or earth sensor), a sun sensor (SSA), and gyros for on-station attitude determination. The beacon sensor provides roll and pitch attitude measurements continuously. The SSA sensor provides yaw (and pitch) attitude measurements when the sun is in the sun sensor field-of-view (FOV). The gyros provide accumulative roll, pitch, and yaw attitude measurements continuously. Typically, beacon sensors consist of a reflector and feed arrays for receiving radio frequency (RF) signals from a beacon ground station on the earth surface, and a beacon sensor processing unit.
Each spacecraft orbit duration is operationally divided into two periods, a “gyro calibration” period and a “gyro compassing” period. Gyro calibration occurs during approximately 4 hours of each orbit, when the sun is in the FOV of the SSA with a favorable geometric dilution factor relative to the beacon sensor. During the gyro calibration period, 3-axis attitude measurements are available, such as the roll and pitch attitude from the beacon and yaw from the SSA. The measured 3-axis attitude is used to update the spacecraft attitude estimate and calibrate the gyro bias and other gyro parameters.
During gyro compassing, which occurs for approximately the remaining 20 hours of each orbit, roll and pitch attitude measurements from the beacon sensor are used to update the spacecraft roll and pitch attitude, however, the yaw attitude is not available. The roll gyro sensor is used to estimate the yaw attitude and maintain yaw pointing by the gyro compassing method. Yaw attitude estimation accuracy by gyro compassing directly depends on the roll gyro bias calibration accuracy in the gyro calibration period and subsequent drift during the gyro compassing period.
Roll gyro bias drift is affected by multiple factors, such as the reflector and sensor thermal distortion, reflector deployment alignment knowledge error (reflector is mechanically co-aligned with the beacon sensor), and time-varying spacecraft steering rate. For a mobile communication satellite with orbital inclination and flying target-normal steering, the steering rate has sinusoidal components in all three axes, which is especially pronounced in the roll and yaw axes. Specifically, the alignment knowledge error of the deployed reflector together with the time-varying body steering rate produces an apparent sinusoidal gyro bias error. This, in turn, causes a corresponding yaw pointing error during the gyro compassing period.
In order to reduce the yaw pointing error, the roll gyro bias error needs to be reduced in the gyro compassing period. One effective way to reduce the roll gyro bias error is to reduce the component of the error caused by the reflector (or beacon) alignment knowledge error. To meet the spacecraft requirements for precision pointing, it has been found that the knowledge error of the reflector alignment should be less than about 0.05° versus the deployment repeatability of 0.4°.
As a result, there is a need for a system and method for reflector deployment error estimation.